docs: def to_ip(self, values, from_unit):
          """Return values in IP and the units to which the values have been converted."""
          if from_unit in self._ip_units:
              return values, from_unit
          elif from_unit == 'degC-hours':
              return self.to_unit(values, 'degF-hours', from_unit), 'degF-hours'
          else:
              return self.to_unit(values, 'degF-days', from_unit), 'degF-days'